Referring to FIGS. 1 and 2, a conventional pneumatic tool disclosed in Taiwanese Utility Model Patent No. M396733 includes a tool body 11 that is formed with a flow channel 12, a valve unit 13 that includes a first valve member 131 and a second valve member 133, and a trigger member 14 that is connected co-movably to the first valve member 131. The first valve member 131 has a valve portion 132. The second valve member 133 is connected movably to the first valve member 131. With the trigger member 14 being pressed to move along an axis (A) from a first position (see FIG. 1) to a second position (see FIG. 2), the valve portion 132 of the first valve member 131 and the second valve member 133 in turn unblock the flow channel 12 to obtain different opening degrees of the flow channel 12. However, a user can hardly perceive opening degree of the flow channel 12 since there is no distinct indication during the movement of the trigger member 14 from the first position to the second position.
Referring to FIG. 3, another convention pneumatic tool 2 includes a tool body 21 and a trigger member 22 connected pivotally to the tool body 21. The trigger member 22 is pressable to drive a valve unit to gradually unblock a flow channel formed in the tool body 21. However, there is no means for indicating opening degree of the flow channel.